1. Field of the invention
The present invention relates to a vari-focal lens system which has a short total length, a high aperture ratio and a high vari-focal ratio, comprises four lens units and is equipped with a rear focusing means.
2. Description of the prior art
Owing to the recent progresses which have recently been made to design compact and light-weight video cameras manufacturable at lower costs, the market of cam coders is very much activated to make the video cameras rapidly popularized among amateur photographers.
A video camera consists mainly of an electrical circuit board, an actuator system (a mechanical system) and an imaging optical system. Though attempts have hitherto been made to design the compact video cameras manufacturable at the lower costs mainly by improving the electrical system, the imaging optical systems are now rapidly made compacter.
Under the present circumstances, the compacter designs and reduction of the manufacturing costs are made by developing new vari-focal type optical systems through effective utilization of the progresses made in the technologies for compacter designs of imagers, shaping of revolutionally symmetrical aspherical lens surface and improvement in TTL type automatic focusing.
As examples of the newly developed vari-focal type optical systems, there are known the lens systems disclosed by Japanese Patents Kokai Publication No. Sho 62-24213, Kokai Publication No. Sho 62-178917, Kokai Publication No. Sho 62-215225 and so on. However, it is limitlessly demanded to obtain compacter and lighter lens systems, especially to shorten total lengths of lens systems and reduce diameters of the first lens units to be used therein.
As the vari-focal lens system which is proposed before the above-mentioned conventional examples, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-186818, which is designed for a vari-focal ratio on the order of 8. This lens system consists, in the order from the object side, of a vari-focal subsystem composed of a first lens unit which is movable for focusing, a second lens unit which has a negative refractive power and is movable for varying focal length, and a third lens unit which has a negative refractive power and is movable for correcting the deviation of image point caused by variation of focal length; and an imaging subsystem composed of an aperture stop and a relay lens unit. In this lens system, the first lens unit consists, in the order from the object side, of a negative lens element, a positive lens element and a positive lens element; the second lens unit consists, in the order from the object side, of a negative lens element, a negative lens element and a positive lens element; and the third lens unit consists, in the order from the object side, of a negative meniscus lens element having a strongly concave surface on the object side.
This lens system has a long total length and uses the first lens unit which has a large diameter. Further, this lens system allows remarkable variation of spherical aberration to be caused in the vicinity of the tele position by moving the first lens unit for focusing, thereby not permitting the first lens unit to have so strong a power. Accordingly, the point of the image formed by the first lens unit, i.e., the object point for the second lens unit is far from these lens units, whereby the second lens unit inevitably has a weak refractive power. As a result, the second lens unit must be moved for a long distance for variation of the focal length of the lens system and prolongs the total length of the vari-focal subsystem. Further, since the first lens unit and the second lens unit have the weak powers respectively, the entrance pupil is far from the first surface of the lens system toward the image side, thereby making it necessary to enlarge the diameter especially of the first lens unit. Furthermore, the convex lens elements to be arranged in the first lens unit must be thick for reserving the required thickness at the marginal portions thereof, thereby producing a vicious cycle to locate the entrance pupil farther from the first surface of the lens system, enlarge the diameter of the first lens unit and prolong the total length of the vari-focal subsystem. In addition, since the lens system is focused by moving the first lens unit, the entrance pupil is brought still farther from the first surface of the lens system when it is focused on an object located at a short distance. In order to solve this problem related to focusing, the above-mentioned Japanese Patents Kokai Publication No. Sho 62-24213, Kokai Publication No. Sho 62-178917, Kokai Publication No. Sho 62-215225 and so on disclosed the vari-focal lens systems of the types which adopted the rear focusing method and/or used a compensator arranged after an aperture stop. These publications provided technical means very excellent for shortening total lengths of vari-focal lens systems and reducing diameters of the first lens units. Especially, Japanese Patent Kokai Publication No. Sho 62-178917 contained technical information which permits remarkably reducing number of lens elements required for composing the lens system and correcting aberrations favorably therein by using an aspherical surface in the imaging subsystem. However, this publication scarcely made use of this technology effectively for designing the lens system, whereby the total length of the lens system disclosed by this publication and the diameter of the first lens unit used in this lens system remained substantially the same as those of the lens system proposed previously.
Speaking concretely, the lens system disclosed by Japanese Patent Kokai Publication No. Sho 62-178917 consists of a vari-focal subsystem which is composed of a first lens unit having a positive refractive power, and a second lens unit having a negative refractive power; and an imaging subsystem which is composed of a third lens unit always kept stationary and a fourth lens unit movable for correcting deviation of the image surface caused by variation of focal length or variation to the distance as measured to an object to be imaged. The third lens unit of this lens system consists only of a single positive lens element, whereas the fourth lens unit consists of two or three lens elements including at least one negative lens element. By adopting the rear focusing method and using the aspherical surface, Japanese Patent Kokai Publication No. Sho 62-178917 succeeded in composing the lens system of less than 10 lens elements or eliminating the unnecessary airspaces, thereby shortening the total length of the lens system and reducing the diameter of the first lens unit.
Though the lens system described above adopts the rear focusing method so as to permit strengthening the power of the first lens unit, the lens system uses the first lens unit which has a power not so strong and the second lens unit whose power is still weak. Further, the third lens unit which is composed of the single lens element produces an undesirable effect that it cannot converge a light bundle sufficiently for composing an afocal lens system and it obliges the fourth lens unit to have a long focal length, thereby making it impossible to shorten the total length of the lens system and sufficiently shorten the diameter of the first lens unit.